How can I setup blender for texture painting?
Texture Painting

If you paint a diffuse map, lights don't really matter that much. You don't render anything, so nothing but the color of your choice influences the texture. 

Any light setup might only influence, how you perceive the colors. 

Lights (for baking):

Delete all lights, you won't need them. Go to the World Properties Tab and change the Background Color to white.



Material Setup (for painting):

Go to the Shader Editor, create a new material and delete the Principled BSDF. Create a new Image Texture and connect it to an Emission Node (with Node Wrangler installed, Ctrl+Shift+LMB) to the Material Output.



Render Engine: 

If you only want to paint textures and you don't intend to bake additional maps (normal, AO, etc.) the Render Engine doesn't really matter. If you do want to bake additional maps, Cycles is the only option, since baking is not possible in Eevee (yet).

Since you paint in Material Preview, you might want to change the HDRi Image in the Studio Lights setup to a more neutral one* and set the Background Visibility to zero.




hdrihaven.com has some nice outdoor HDRis with overcast sky that deliver a nice flat shading. They still influence your color perception, though...